The stratum corneum from animal skin arises as a result of a programmed differentiation of underlying epidermal keratinocytes. The terminal events involved in the formation of this non-living structure from living stratum granulosum cells are complex and the molecular mechanisms are incompletely understood. The process involves nuclear explusion, externalization of lipid-containing lamellar bodies, enzyme-catalyzed degradation and selective loss of all major classes of macromolecules and cellular dehydration. The dehydrated cells become flattened and they adhere tightly to form the final laminated structure which provides a selective barrier to the entry or exit of substances to or from the skin.
Numerous lines of investigation have led to the principle that the extent of hydration of the stratum corneum is responsible for the clinical feel and appearance of dry skin. The stratum corneum from normal skin contains 10-20% water, whereas the stratum corneum from clinically dry skin contains 5-10% water. Dry stratum corneum can be treated by direct addition of water, by occlusion to prevent water evaporation or by addition of substances with humectant (i.e. water-retaining) activity. Humectants commonly used in skin moisturizing products include glycerol, urea, propylene glycol, mineral oil and pyroolidine carboxyllic acid. More recently, naturally-occurring macromolecules such as collagen, hyaluronic acid, elastin or placental proteins have been used as humectants. Although all these substances are reasonably good humectants, it is clear that none of them are involved in the natural mechanisms of stratum corneum moisturization. We have identified and described a class of proteolipids from epidermis that represents at least one of the natural molecules whose function is to hydrate the stratum corneum. Proteolipids are ideally suited for this function. Because of their amphiphatic nature, the lipid portion of these "hybrid" molecules should bind to the hydrophobic stratum corneum, leaving the hydrophilic protein portion free to bind water.
A class of compounds, lipoproteins, is known. However compounds of this class differ from proteolipids in the following respect. Lipoproteins are a loose combination of a protein and a fatty acid held together by electrostatic attraction (such as a salt) and therefore the protein and lipid parts separate in aqueous solution. Proteolipids, on the other hand, consist of a protein and fatty acid joined by a covalent bond, the exact nature of which is not yet known. Thus the components of a proteolipid do not separate in aqueous solution and the proteolipid exists as such in water.